The invention pertains to a composition comprising silver metal particles and an additive, to thermally resistant conductive layers, to an active matrix liquid crystalline display (AMLCD) comprising the said layers, and to the use of said composition in the manufacture of silver-containing layers in an article.
Compositions comprising silver particles with or without other constituents, are well known in the art. For instance, in EP 826,415 a method for preparing a silver sol has been disclosed. These silver sols are used for making a conductive film on a substrate. By spin-coating the sol onto a substrate, such films are made and these are then heated at 150° C.
In EP 276,459 a method for manufacturing cathode-ray tubes was disclosed. Antistatic silicon dioxide films were prepared that contained small amounts of (among others) silver. To a solution or a colloidal solution containing metal particles a cationic or anionic surfactant was added as an additive for improving the stability of the solution, after which an antistatic film was produced by a spraying, dispensing, or dipping method of this material onto a substrate, followed by heating at 200° C. for 15 min.
It was found that silver-containing layers, particularly layers that contain 80 vol. % or more silver, cannot be heated above 250° C., and preferably not above 200° C. without serious appearance of irreversible exfoliating and/or creep phenomena. Creep is a process wherein the film deteriorates into a plurality of small sections containing silver and section therein between containing no silver. On creep the surface roughens, which means that the mirror-like appearance of the silver-containing layer disappears. Silver-containing layers (or films) that have undergone exfoliation and/or creep no longer have a low resistivity, and because of the low conductivity have become unsuitable for most of the applications that require a resistivity of less than 6 μΩ.cm (microOhm.centimetcr). Since silver-containing layers are usually made of compositions also comprising organic materials such as organic binder materials, and because only relatively low temperatures (less than 250° C., preferably less than 200° C.) can be used for making silver-containing layers with sufficient conductivity, such layers usually contain amounts of organic materials that are not sufficiently removed at those temperatures. Also in further processing steps high temperatures (higher than 250° C.) may be necessary. For instance, flitting a CRT-cone to a screen occurs at 450° C. To make it possible to use silver at such high temperatures, a polymeric binder and frit glass particles can be mixed into a silver paste. However, the presence of the glass particles reduces the conductivity considerably. Furthermore, since these added particles have sizes in the micron range, such mixed pastes are no longer suitable to make silver-containing layers thinner than 1 μm. Thin layers of well-conducting silver on an insulating surface, such as on Coming®1737 glass, which are used for active matrix liquid crystal displays (AMLCD), or on glass or any other substrate for use in infrared reflective stacks, can be made by using an electroless silver process, but the maximum temperature in the further reaction steps is limited to 250° C. At that temperature Si3N4 deposition takes place, and moreover, the electroless process is commercially not desired anyway since it is a slow non-equilibrium process with a short bath pot life.
There is a considerable need for a method of making thin conductive silver-containing layers that can be heated at temperatures higher than 250° C. without losing its conductivity and mirror-like appearance.